1. Field of the Invention
The present invention relates to an LCD drive circuit formed integrally with a liquid crystal display panel and more particularly to an LCD drive circuit suitable for use in a TFT-LCD (Thin Film Transistor-Liquid Crystal Display) circuit.
2. Description of the Prior Art
In the field of TFT-LCD, attempts have heretofore been made to incorporate a drive circuit in the body of an LCD.
That is because the drive circuit for driving an LCD is generally fabricated as an integrated circuit and is disposed around the LCD. In other words, the reason for making the aforesaid attempts is that a display panel or the like using an LCD assumes a picture frame-like shape and the actual display area of the LCD is narrowed.
According to a method for solving the above-mentioned problem, a shift register constituted by an n-channel FET (Field Effect Transistor) for example is formed as an LCD drive circuit on the glass substrate which constitutes the LCD.
According to a broad classification of such n-channel FET shift registers, the following shift registers are conceivable.
FIGS. 8A and 8B show configuration examples of n-channel shift registers. First, the shift register shown in FIG. 8A is constituted by a ratio circuit.
In the shift register shown in FIG. 8A, clock input terminals .phi..sub.1 and .phi..sub.-1 supply clock signals of opposite phases. Consequently, with .phi..sub.1 at "H" (high level), the value of an input terminal D is read in, while when .phi..sub.-1 is at "H," the state of the input terminal D which has been read in appears at an output terminal Q. In the same figure, Vd denotes a drain voltage (power source).
In the above ratio circuit, however, a steady-state through current flows through the circuit, so that the power consumption increases, which is an obstacle to the reduction in size of the circuit.
On the other hand, the shift register shown in FIG. 8B is constituted by a ratioless circuit. In the shift register shown therein, pulse type clock signals of different phases as shown in FIG. 8C are applied to clock input terminals .phi..sub.1 to .phi..sub.4. In this circuit, with .phi..sub.1 at "H," C is charged; with .phi..sub.2 at "H," the value of an input terminal D is read in; and with .phi..sub.4 at "H," the state of the input terminal D which has been read in is reflected in an output terminal Q.
However, the above ratioless circuit is disadvantageous in that a pulse is mixed into the output signal in order to maintain the operation of the circuit.
For eliminating such a drawback it is required to connect a static inverter as a buffer to the output terminal Q, but an increase of power consumption results.
As a display device which replaces a CRT (Cathode Ray Tube), the TFT-LCD is in many cases applied to a device which displays an image on the basis of a so-called television signal (a composite signal having both a luminance signal and a synchronizing signal).
Heretofore, in the television signal a vertical retrace interval proportional to a vertical synchronizing timing and a horizontal retrace interval proportional to a horizontal synchronizing timing have been present in association with both the scanning characteristic and decay characteristic of the CRT.
In Japanese Published Unexamined Patent Application No. Hei 6-337655 (1994) is disclosed a technique such that in both retrace periods mentioned above the operation of all buffers each inserted in the output terminal of a shift register is stopped to diminish the power consumption of the LCD.
However, even with the technique disclosed in the above unexamined publication, the reduction in power consumption of the LCD drive circuit can be expected only 10% to 20%.